The present invention relates to a spread spectrum receiver having a synchronous tracking section for synchronously tracking a spread signal by correlating a phase-lead spread signal and a phase-lag spread signal by use of time division to thereby produce an error signal, and more particularly, to a spread spectrum receiver which enables reliable and highly accurate synchronous tracking and synchronous holding by simplified control of timing of a synchronous tracking loop and which affords high flexibility in terms of system design.
Rapid leads have been made in mobile communications technology, typified by a portable cellular phone compliant with a spread spectrum communication (hereinafter often abbreviated as xe2x80x9cSS communicationxe2x80x9d) system or a code division multiple access (i.e., CDMA) communications system. In such a communications system, synchronization used for extracting and holding a required timing signal from a received signal is indispensable for correctly reproducing received data through demodulation and despreading of a spectrum spread (SS) signal.
Synchronization of a carrier wave and data is required for receiving the spectrum spread signal, as in the case with receipt of another digital signal. Synchronization of a spread signal is critical, thus imposing a unique problem. Since a spread spectrum signal whose energy is spread over a wide frequency range is used in the spread spectrum communications system, a signal-to-noise (S/N) ratio is considerably low. Particularly, in the CDMA communications system, another channel signal is transmitted at the same frequency as is a desired signal, and hence a signal-to-interference (S/I) ratio is considerably low. For this reason, the S/N ratio or the S/I characteristic must be improved by correctly despreading a received spread spectrum signal. As a result of correct despreading of the received spread spectrum signal, a conventional technique used for demodulating an ordinary signal can be applied to synchronization of a carrier wave and data.
For these reasons, in the spread spectrum communications system, synchronization is carried out in two steps by use of two independent circuit means. One step is a xe2x80x9csynchronization acquisitionxe2x80x9d step in which timing information is narrowed down to predetermined uncertainty, e.g., an extent substantially equal to the time duration of a chip, by observation of a received signal, and the timing at which the receiver activates a spread system is matched to the timing of the received signal. Another step is xe2x80x9csynchronization trackingxe2x80x9d for more accurately holding the timing that is obtained through synchronization acquisition so as to prevent the spread system of the receiver from causing a timing lag.
To correctly carry out synchronization tracking, a timing error of the received signal must be restrained within a certain extent through synchronization acquisition. Since synchronization acquisition requires high-speed characteristics, an open loop is primarily employed. In contrast, since synchronization tracking imposes special emphasis on accuracy, observation must be performed for a comparatively long period of time in order to reduce the influence of noise. For this reason, a closed loop is primarily used.
FIG. 4 is a circuit configuration of a synchronization tracking section in a RAKE receiver, which is one example of a conventional spread spectrum receiver. A xcfx84-dither loop is used as the synchronization tracking section of the RAKE receiver, and this xcfx84-dither loop constitutes a closed loop employing one set of correlators (despreading sections). Taking note that an autocorrelation function assumes a triangular shape in the vicinity of a correct point of synchronization, the xcfx84-dither loop causes the phase of a spread signal (or a pseudo noise system code) produced by the receiver to slightly lead the point of synchronization and slightly lag the point of synchronization, extracts variations in a correlation value caused by the phase lead and phase lag of the spread signal relative to the point of synchronization (i.e., variations in the amplitude of the signal that has been despread), and controls the phase of oscillation of the entire synchronization tracking section so as to reduce the variation to zero.
In FIG. 4, the synchronization tracking section of the conventional spread spectrum receiver comprises a sampling section 101, a despread section 102, an integration section 103, a lead/lag phase level difference output section 409, a pseudo noise (PN) code generator 111, and a timing control section 412.
The sampling section 101 samples a received signal RxI[i-1:0], which is received byway of an unillustrated antenna and is in phase with a received carrier wave, as well as a received quadrature signal ExQ[i-1:0], which differs in phase from the carrier wave by 90xc2x0, at the timing of an ELCLK signal output from the timing control section 412. Each of the received signals RxI and RxQ has a duration of i-bits. To clarify the bit configuration of a signal, the received signals are represented as RxI[i-1:0] and RxQ[i-1:0].
Signals ELI[i-1:0] and ELQ[i-1:0] resulting from sampling of the received signals RxI[i-1:0] and RxQ[i-1:0] by the sampling section 101 are despread by the despread section 102 through correlation of the PN system codes PnI and PnQ that are output from the pseudo noise code generation section 111 and serve as spread signals. The despread section 102 outputs to the integration section 103 correlation data PELI[j-1:0] and PELQ[j-1:0], each correlation data set comprising j-bits. According to a signal PNc which is output from the timing control section 412 and represents whether the spread signal is in a lead phase or a lag phase, the PN code generation section 111 supplies the PN system codes PnI and PnQ, each of which is in a lead phase or a lag phase, to the despread section 102 by use of time division.
On the basis of a signal EL1 which is supplied from the timing control section 412 and indicates whether the lead-phase or lag-phase correlation data is computed, as well as on the basis of an Integ signal representing an integration number, the integration section 103 integrates the correlation data PELI[j-1:0] and PELQ[j-1:0] thereby outputting integration data PELICc[k-1:0] and PELQCc[k-1:0], each integration data set comprising k-bits, to the delay circuit 108 which delays the lead/lag phase level difference output section 409 and the integration data by one symbol. More specifically, the delay circuit 108 outputs delayed integration data PELId[k-1:0] and PELQd[k-1:0] each comprising k-bits, that are formed by delaying the integration data PELICc[k-1:0] and PELQCc[k-1:0] by one symbol.
The lead/lag level difference output section 409 outputs a level difference (i.e., an error signal) between the lead phase and the lag phase to the timing control section 412, on the basis of the integration data PELICc[k-1:0] and PELQCc[k-1:0], the delayed integration data PELId[k-1:0] and PELQd[k-1:0), and the signal EL2 representing whether the current symbol corresponds to lead phase or lag phase.
The timing control section 412 controls the phase of the entire synchronization tracking section by controlling sampling timing (i.e., the timing of the ELCLK signal) and the timing at which each of the signals PNc, EL1, and EL2 is switched between lead phase and lag phase, so as to reduce the level difference TAC to zero.
However, in the synchronization tracking section of the foregoing conventional spread spectrum receiver, the signal EL1xe2x80x94which represents whether or not the integration section 103 integrates a lead-phase or lag-phase correction valuexe2x80x94and the signal EL2xe2x80x94which represents the integration data output from the integration section 103 are in a lead phase or a lag phasexe2x80x94are not input to the lead/lag phase level difference output section 409 at the same timing. No problem arises so far as the correlation between the two signals ELl and EL2 is correctly maintained. However, if for any reason the correlation becomes offset, tracking for the purpose of further improving the synchronization of the spread signal (i.e., the PN system code) roughly captured by the search circuit becomes impossible, which in turn makes holding of synchronization impossible.
The constituent elements of the synchronization tracking section shown in FIG. 4 are embodied by circuits and a digital signal processor (DSP). There is a time lag between the timing at which the sampling timing signal ELCK is output from the sampling section 101 and the timing at which the lead/lag phase level difference output section 409 performs processing operations. Timings of these two signals EL1 and EL2 must be set so as to comprise the time lag. The requirement for strict management of the individual processing timings of the loop makes system design inflexible.
The present invention has been conceived in light of the foregoing problems and circumstances, and the object of the present invention is to provide a spread spectrum receiver having a synchronization tracking section which performs a correlation operation, by use of time division, for generating an error signal between a phase-lead spread signal and a phase-lag spread signal, wherein timing control required by a loop to perform synchronization tracking such as setting of timing of a signal which indicates whether a signal supplied to constituent elements for performing integration and level difference operations is in a lead phase or a lag phase is further simplified, thereby enabling highly accurate and reliable synchronization tracking and holding and affording a high degree of flexibility in terms of system design.
According to a first aspect of the present invention, there is provided a spread spectrum receiver having a 10 synchronization tracking section for tracking synchronization between a phase-lead spread signal and a phase-lag spread signal through correlation of the spread signals by use of time division so as to generate an error signal between the two spread signals, the spread spectrum receiver comprising: despread means for despreading received signals through correlation; integration means for integrating the signal output from the despread means; lead/lag information addition means for adding, to the signal received from the integration means, lead/lag information representing whether the signal output from the integration means is in a lead phase or a lag phase; delay means for delaying, by one symbol, the signal received from the lead/lag information addition means; lead/lag level difference output means for outputting a level difference between lead phase and lag phase on the basis of the signal output from the lead/lag information addition means and the signal output from the delay means; and timing control means for controlling the phase of the entire loop so as to eliminate the level difference.
In the spread spectrum receiver according to the present invention, the despread means despreads, through correlation, two phase spread signals; i.e., a lead-phase signal and a lag-phase signal. The integration means integrates the signal output from the despread means, and the lead/lag information addition means adds, to the signal output from the integration means, lead/lag information representing whether the signal output from the integration means is in a lead phase or a lag phase. The delay means delays, by one symbol, the signal output from the lead/lag information addition means. The lead/lag level difference output means outputs a level difference between lead phase and lag phase on the basis of the signal output from the lead/lag information addition means and the signal output from the delay means. Further, the timing control means controls the phase of the entire loop so as to eliminate the level difference, thus tracking synchronization of the spread signals.
As mentioned above, the lead/lag information representing whether the signal output from the integration section is in a lead phase or a lag phase is added to the integration data and the delayed integration data supplied to the lead/lag level difference output means. In contrast with the conventional spread spectrum receiver, the spread spectrum receiver according to the present invention eliminates a necessity for the timing control means to control the timing of the lead/lag level difference output section in consideration of a lag of a signal and a processing lag, which would otherwise be required by the conventional synchronization tracking section. Control of timing of the synchronization tracking loop is made simpler, thus enabling reliable and highly accurate tracking and holding of synchronization of the spread signal without involving erroneous operations. Further, the present invention diminishes the necessity for strict management of individual processing timings in the synchronization tracking loop, thus affording a high degree of flexibility in terms of system design.
In a case where a multiple-frequency shift-keying method is employed as a modulation method, two methods are conceivable; i.e., a method by which the lead/lag information addition means adds, to integration data which is one phase component relative to a carrier wave, typical information representing whether integration data are in a lead phase or a lag phase, and a method by which the lead/lag information addition means adds, to integration data which are all phase components relative to a carrier wave, typical information representing whether integration data are in a lead phase or a lag phase. If a typical quadrature phase shift keying (QPSK) is employed, there is employed one of the spread spectrum receivers as defined in the second, third, and fourth aspects (which will be mentioned bellow).
According to a second aspect of the present invention, there is provided a spread spectrum receiver having a synchronization tracking section for tracking synchronization between a phase-lead spread signal and a phase-lag spread signal through correlation of the spread signals by use of time division so as to generate an error signal between the two spread signals, the spread spectrum receiver comprising: despread means for despreading, through correlation, a received signal which is in phase with a carrier wave and a received quadrature signal which differs in phase from the carrier wave by 90xc2x0; integration means for integrating both the in-phase and quadrature signals output from the despread means; lead/lag information addition means for adding, to the in-phase signal output from the integration means, lead/lag information representing whether the signal output from the integration means is in a lead phase or a lag phase; delay means for delaying, by one symbol, the quadrature signal output from the integration means and the signal output from the lead/lag information addition means; lead/lag level difference output means for outputting a level difference between lead phase and lag phase on the basis of the quadrature signal output from the integration means, the signal output from the lead/lag information addition means, and the signal output from the delay means; and timing control means for controlling the phase of the entire loop so as to eliminate the level difference.
In the spread spectrum receiver as defined in the second aspect of the present invention, despread means despreads, through correlation, a received signal which is in phase with a carrier wave and a received quadrature signal which differs in phase from the carrier wave by 90xc2x0; integration means integrates both the in-phase and quadrature signals output from the despread means; lead/lag information addition means adds, to the in-phase signal output from the integration means, lead/lag information representing whether the signal output from the integration means is in a lead phase or a lag phase; delay means delays, by one symbol, the quadrature signal output from the integration means and the signal output from the lead/lag information addition means; lead/lag level difference output means outputs a level difference between lead phase and lag phase on the basis of the quadrature signal output from the integration means, the signal output from the lead/lag information addition means, and the signal output from the delay means; and timing control means controls the phase of the entire loop so as to eliminate the level difference, thus tracking synchronization of the spread signals. As a result, control of timing of the synchronization tracking loop is made simpler, thus enabling reliable and highly accurate tracking and holding of synchronization of the spread signal without involving erroneous operations. Further, the present invention diminishes the necessity for strict management of individual processing timings in the synchronization tracking loop, thus affording a high degree of flexibility in terms of system design.
According to a third aspect of the present invention, there is provided a spread spectrum receiver having a synchronization tracking section for tracking synchronization between a phase-lead spread signal and a phase-lag spread signal through correlation of the spread signals in a time-division manner so as to generate an error signal between the two spread signals, the spread spectrum receiver comprising: despread means for despreading, through correlation, a received signal which is in phase with a carrier wave and a received quadrature signal which differs in phase from the carrier wave by 90xc2x0; integration means for integrating both the in-phase and quadrature signals output from the despread means; lead/lag information addition means for adding, to the quadrature signal output from the integration means, lead/lag information representing whether the signal output from the integration means is in a lead phase or a lag phase; delay means for delaying, by one symbol, the in-phase signal output from the integration means and the signal output from the lead/lag information addition means; lead/lag level difference output means for outputting a level difference between lead phase and lag phase on the basis of the in-phase signal output from the integration means, the signal output from the lead/lag information addition means, and the signal output from the delay means; and timing control means for controlling the phase of the entire loop so as to eliminate the level difference.
In the spread spectrum receiver as defined in the third aspect of the present invention, despread means despreads, through correlation, a received signal which is in phase with a carrier wave and a received quadrature signal which differs in phase from the carrier wave by 90xc2x0; integration means integrates both the in-phase and quadrature signals output from the despread means; lead/lag information addition means adds, to the quadrature signal output from the integration means lead/lag information representing, whether the signal output from the integration means is in a lead phase or a lag phase; delay means delays, by one symbol, the in-phase signal output from the integration means and the signal output from the lead/lag information addition means; lead/lag level difference output means outputs a level difference between lead phase and lag phase on the basis of the in-phase signal output from the integration means, the signal output from the lead/lag information addition means, and the signal output from the delay means; and timing control means controls the phase of the entire loop so as to eliminate the level difference, thus enabling reliable and highly accurate tracking and holding of synchronization of the spread signal without involving erroneous operations. Further, the present invention diminishes the necessity for strict management of individual processing timings in the synchronization tracking loop, thus affording a high degree of flexibility in terms of system design.
According to a fourth aspect of the present invention, there is provided a spread spectrum receiver having a synchronization tracking section for tracking synchronization between a phase-lead spread signal and a phase-lag spread signal through correlation of the spread signals by use of time division so as to generate an error signal between the two spread signals, the spread spectrum receiver comprising: despread means for despreading, through correlation, a received signal which is in phase with a carrier wave and a received quadrature signal which differs in phase from the carrier wave by 90xc2x0; integration means for integrating both the in-phase and quadrature signals output from the despread means; lead/lag information addition means for adding, to the in-phase and quadrature signals output from the integration means, lead/lag information representing whether the signal output from the integration means is in a lead phase or a lag phase; delay means for delaying, by one symbol, the signal output from the lead/lag information addition means; lead/lag level difference output means for outputting a level difference between lead phase and lag phase on the basis of the signal output from the lead/lag information addition means and the signal output from the delay means; and timing control means for controlling the phase of the entire loop so as to eliminate the level difference.
In the spread spectrum receiver according to the fourth aspect of the present invention, despread means despreads, through correlation, a received signal which is in phase with a carrier wave and a received quadrature signal which differs in phase from the carrier wave by 90xc2x0; integration means integrates both the in-phase and quadrature signals output from the despread means; lead/lag information addition means adds, to the in-phase and quadrature signals output from the integration means, lead/lag information representing whether the signal output from the-integration means is in a lead phase or a lag phase; delay means delays, by one symbol, the signal output from the lead/lag information addition means; lead/lag level difference output means outputs a level difference between lead phase and lag phase on the basis of the signal output from the lead/lag information addition means and the signal output from the delay means; and timing control means controls the phase of the entire loop so as to eliminate the level difference, thus enabling reliable and highly accurate tracking and holding of synchronization of the spread signal without involving erroneous operations. Further, the present invention diminishes the necessity for strict management of individual processing timings in the synchronization tracking loop, thus affording a high degree of flexibility in terms of system design.
According to a fifth aspect of the present invention, the spread spectrum receiver preferably comprises sampling means for sampling the received signal at predetermined sampling timing; the despread means preferably despreads the thus-sampled received signal; and the timing control means preferably controls the sampling timing according to the level difference.
In the spread spectrum receiver according to the fifth aspect of the present invention, it is preferable for the sampling means to sample the received signal at predetermined sampling timing; the despread means to despread the thus-sampled received signal; and the timing control means to control the sampling timing according to the level difference.
According to a sixth aspect of the present invention, in the spread spectrum receiver, the spread signal corresponds to a pseudo noise code. The spread spectrum receiver preferably comprises pseudo noise code generation means for supplying to the despread means, by use of time division, two pseudo noise codes; namely, a phase-lead pseudo noise code and a phase-lag pseudo noise code, according to a lead/lag signal representing whether the signal is in a lead phase or a lag phase. The timing control means controls the timing at which the lead/lag signal is switched according to the level difference.
In the spread spectrum receiver according to the sixth aspect of the present invention, the spread signal preferably corresponds to a pseudo noise code. Preferably, pseudo noise code generation means supplies to the despread means, by use of time division, two pseudo noise codes; namely, a phase-lead pseudo noise code and a phase-lag pseudo noise code, according to a lead/lag signal representing whether the signal is in a lead phase or a lag phase. Preferably, the timing control means controls the timing at which the lead/lag signal is switched according to the level difference. The expression xe2x80x9cpseudo noise codexe2x80x9d designates a xe2x80x9cpseudo noise code in a broad sensexe2x80x9d and covers various code systems such as the M code system and the Gold code system.